Manufacturing of integrated circuits is generally a procedure of forming thin films and layers of various materials on wafers of base semiconductor material, and selectively removing areas of such films to provide structures and circuitry. Doped silicon is a typical base wafer material. CVD is a well known process for depositing such thin films and layers. For example, polysilicon may be deposited from silane gas, SiH.sub.4. It is known, too, to deposit tungsten silicide from a mixture of gases including silane and a tungsten-bearing gas such as tungsten hexaflouride. Pure tungsten is also deposited on silicon wafers in the manufacture of integrated circuits, sometimes selectively and sometimes across the entire surface in a process known as "blanket" tungsten.
In a typical CVD process wafers are placed on supports within a sealable chamber, the chamber is sealed and evacuated, the wafers are heated, typically by heating the wafer support, and a gas mixture is introduced into the chamber. For example, in the blanket tungsten process, tungsten hexaflouride and hydrogen are introduced as reactive gases and argon may be introduced as a non-reactive carrier gas. The tungsten hexaflouride is the source of deposited tungsten. Typically the gases are flowed continuously during processing. The temperature of a substrate (wafer) to be coated is one of the variables that drives the chemical reaction to cause tungsten to be deposited on the wafer surface. It is important to control the temperature, the concentration of various gases in the mixture introduced, and such characteristics as the uniformity of flow of gas over the surface being coated, among other variables. An even thickness of a deposited layer is an important characteristic.
A common arrangement in a CVD processing apparatus is to place a wafer against a flat surface, such as a surface on a central turret in a chamber that can be evacuated and into which CVD gases may be introduced. The turret is heated to heat the wafer. It is also known to support a wafer on a CVD chuck separate from but attached to a central turret, and to heat the chuck to heat the wafer. This arrangement allows for a lower thermal mass for the chuck and consequently a quicker response time when it is necessary to change the temperature.
A relatively common arrangement in CVD processing is to provide several heatable chucks connected to a central turret within a sealable chamber that may be evacuated, and to which processing gases may be introduced. Wafers are placed on the chucks, the chamber is sealed and evacuated, and process gases are introduced. Deposition takes place on the exposed surface of the wafers on the chucks from heat-precipitated chemical reaction, the deposited material being contributed by one of the gases introduced into the sealed chamber.
In apparatus like that described above, it is now common to transfer wafers to and from the CVD processing chamber through vacuum load and unload locks, which allows the processing chamber to remain under vacuum through the processing of several "batches" of wafers, a "batch" being typically equal to the number of chucks available for wafer support in the processing chamber.
Allowing the processing chamber to remain in a relative vacuum rather than being exposed to air has a distinct advantage in that the process conditions may be re-established relatively quickly after one batch of coated wafers is unloaded and another batch of wafers to be coated is loaded to the chucks.
One reason exposure to air can be detrimental is that the gas constituents in air may contaminate the chamber between processing cycles. Oxygen may be particularly troublesome in some processes. Another reason is that air molecules tend to adhere to surfaces, to be absorbed in materials within the chamber, and to become trapped in remote regions like screw threads. Vacuum pumpdown after exposure to air is a relatively long process compared to re-establishing process conditions in a chamber that has not been exposed to air. Use of load and unload air locks, allowing the chamber to remain under vacuum during wafer unload and load operations, allows higher throughput of processed parts, hence greater return on investment.
Still, there are definite limits to the number of batches that can be processed before an air-locked chamber is exposed to air. One reason is that not all the deposition is on the exposed surface of the wafer. The coating material is typically deposited on all surfaces in the CVD chamber that are at a sufficient temperature to precipitate the deposition. Although an effort is made to minimize heated surfaces in the chamber by providing heated chucks, it is typically not feasible to heat only the wafer. Material deposits on the chuck surfaces away from the wafer.
The processed wafers are removed after a batch run, and uncoated wafers are loaded to the chucks. The chucks, however, undergo an accumulative deposition. After a certain amount of accumulation of material, depending on such things as the material deposited, the material of the chuck, temperature variations for the apparatus, and other variables, the accumulation of material on exposed chuck surfaces may begin to peel and flake. If peeling and flaking occurs, the resulting particles can be very detrimental to the substrates (wafers) being coated and to operation of Precision equipment. Particles are particularly troublesome if they form at or very close to a wafer in process, because then they are more likely to adhere to the wafer or to become incorporated in a film being deposited on the wafer.
A typical procedure to combat formation of particulates due to coating buildup is to periodically vent the chamber and clean accumulated deposits from the surfaces in the chamber. A cleaning operation typically requires a subsequent long period of pumpdown and reconditioning to bring the process chamber back to operating condition.
It is also known to plasma etch the chamber to remove offending material accumulation before it becomes a source of generated particles, but this procedure has its own set of difficulties. Besides requiring introduction of etching species (gas), which may be contaminating to subsequent processing cycles, expensive plasma generating apparatus is also needed. There is also the time involved in performing the plasma etch operations, and the problem of ascertaining when the cleaning is finished. Moreover, plasma cleaning often has an effect of removing material from one area but depositing it on another.
Another problem common to most CVD operations in the manufacture of ICs is contamination. One group of contaminants of concern is the column of elements in the periodic table that includes chlorine and fluorine. These elements are in general very mobile, even in solids, and very chemically reactive. Chlorine and fluorine are particular problems, because these elements are present in some of the gases commonly used for providing material to be deposited. For example, flourine is a constituent of WF.sub.6, which is the most common process gas used to supply tungsten to make tungsten films in CVD processing. Other gases provide chlorine, such as dichlorosilane, a gas sometimes used in processes requiring silicon.
Besides being primary contaminants, gases like chlorine and flourine are known to react with chuck materials and to "leach" material from the chuck. One of the more popular chuck materials is Monel metal, which has copper as a constituent. Chlorine in particular is known to be a problem with monel, causing contamination of wafers with copper.
The inventors have experimented with interchangeable chuck faces to combat the above problems, and encountered considerable difficulty. One problem is that a chuck face that is not an integral part of the chuck has to come to a surface temperature sufficient to bring the wafer that it supports to processing temperature. The extra physical interface means that the chuck itself has to be hotter than before. Accomplishing the purpose without overheating and damaging the chuck is a particular problem.
In attempting to lessen the affect of the extra interface it has been found that fastening the removable face securely to the chuck, such as with conventional screw fasteners, is helpful for heat transfer, but detrimental in that the expansion characteristics, if the materials are different, often causes damage. Fastening also presents a problem of removal and remounting when it is time to service the system by replacing the chuck faces.
What is clearly needed to combat these problems is a system of interchangeable chuck faces made of a material that eliminates the leaching phenomenon, and allows quick change. Interchangeable faces also need to be fabricated and mounted in a way that the face temperature to heat the wafer is adequate while avoiding overheating of the chuck itself. In addition, fasteners must be avoided to provide the quickest change and to eliminate breakage and damage due to expansion and contraction. The faces still need to be secure to the chucks in operation.